1. Field of the Invention
The present invention relates to a laser-type gas analyzer that measures, by laser light, the gas concentration of various types of gas in flues.
2. Related Art
As is known, gas atoms and molecules have unique optical absorption spectra. For instance, FIG. 20 illustrates the optical absorption spectrum of ammonia (NH3). The abscissa axis of the graph represents wavelength, and the ordinate axis represents optical absorption intensity.
Laser-type gas analyzers are known as gas analyzers that detect the concentration of various types of gas by relying on such optical absorption spectra. In such analyzers, light emitted by a laser light source having an emission wavelength region identical to that of the optical absorption spectrum of the gas to be measured is irradiated to the gas to be measured, and the concentration of the gas is measured using the absorption of laser light by the molecules and atoms of the gas to be measured.
In a gas analyzer that utilizes laser light, the gas concentration is measured on the basis of the principle whereby optical absorption intensity at a specific wavelength is proportional to gas concentration. The attenuation amount of an absorption line at a central wavelength λc is proportional to the gas concentration. Therefore, the gas concentration can be estimated by irradiating the gas with semiconductor laser light having an oscillation wavelength at λc, and by multiplying the corresponding attenuation amount by an appropriate coefficient.
Concentration measurement methods by gas analysis relying on laser light include mainly differential absorption methods and frequency modulation methods. In a differential absorption method, gas concentrations can ordinarily be measured by way of a comparatively simple configuration. In a frequency modulation method, by contrast, signal processing is complex, but it is possible to measure gas concentration with high sensitivity.
For instance, Patent literature 1 (Japanese Patent Application Publication No. H07-151681, title of the invention “Gas Concentration Measurement Device”) discloses a device that measures gas concentration in accordance with a differential absorption method. As illustrated in FIG. 8 of Patent literature 1, this gas concentration measurement device is provided with a two-wavelength type semiconductor laser, a gas cell, a light-receiving lens, a light-receiving unit and a gas concentration measurement device.
As illustrated also in the concentration measurement principle according to a differential absorption method of FIG. 21, a gas is irradiated with two kinds of laser light, namely laser light the oscillation wavelength whereof is set to the central wavelength λc of the respective absorption line, and laser light the oscillation wavelength whereof is set to the central wavelength λr of a wavelength without absorption line; then, a signal intensity difference obtained in the form of the difference in the intensities of signals outputted by respective light-receiving units is converted to concentration through multiplication by appropriate proportionality constants.
For instance, Patent literature 1 above discloses also a device that measures gas concentration according to a frequency modulation method. As illustrated in FIG. 7 of Patent literature 1, this gas concentration measurement device is provided with a frequency modulation-type semiconductor laser, a gas cell, a light-receiving lens, a light-receiving unit and a gas concentration measurement device.
As illustrated in the concentration measurement principle according to a frequency modulation method in FIG. 22, the output of a semiconductor laser is frequency-modulated at a central wavelength λc with a modulation frequency fm, and is irradiated to a gas to be measured, as a target. Absorption lines of gases behave substantially as a quadratic function with respect to frequency. Therefore, the absorption lines fulfill the role of a discriminator, and a signal (second harmonic signal) of a frequency twice the modulation frequency fm is obtained in the light-receiving unit. It becomes thus possible to estimate a fundamental wave by amplitude modulation, through envelope detection in the light-receiving unit, and to obtain a value proportional to gas concentration, through phase synchronization of the ratio of the amplitude of the fundamental and the amplitude of the second harmonic.
For instance, FIG. 23 illustrates a laser-type gas analyzer as a conventional gas analyzer that utilizes laser light. This laser-type gas analyzer is disclosed in Patent literature 2 (Japanese Patent Application Publication No. 2009-47677, title of the invention “Laser-Type Gas Analyzer”).
In FIG. 23, the reference symbols 101a, 101b denote flue walls between which a gas to be measured flows. A light-emitting unit flange 201a and a light-receiving unit flange 201b are respectively disposed, opposing each other, on the flue walls 101a, 101b. 
A light-emitting unit housing 203a is attached to the light-emitting unit flange 201a via a mounting bracket 202a. Optical components such as a laser light source 204, a collimating lens 205 or the like, are built into the light-emitting unit housing 203a. A light-receiving unit housing 203b is attached to the light-receiving unit flange 201b via a mounting bracket 202b. A lens 206, a light-receiving element 207, and a light-receiving unit circuit board 208 that processes output signals of the light-receiving element 207 are built into the light-receiving unit housing 203b. 
In the above configuration, laser light emitted by the laser light source 204 is irradiated into the flue interior, as the space to be measured, and is received by the light-receiving element 207 inside the light-receiving unit housing 203b that is disposed opposing the laser light source 204.
In such light reception, laser light becomes absorbed when gas to be measured is present in the flue interior. Therefore, a light reception signal processing circuit on the light-receiving unit circuit board 208 calculates the concentration of the gas to be measured by relying on the correspondence between optical absorption and the concentration of the gas to be measured.
Patent literature 1: Japanese Patent Application Publication No. H07-151681 (Title of the invention “Gas Concentration Measurement Device”, for instance paragraphs [0004], [0030]; FIG. 7 and FIG. 8)
Patent literature 2: Japanese Patent Application Publication No. 2009-47677 (Title of the invention “Laser-Type Gas Analyzer”, for instance, paragraphs [0029]-[0038]; FIG. 1 to FIG. 7)
Regulations concerning marine exhaust gas have become stricter in recent years. Regulations on SOx, specifically, require that the criteria of Expression 1 below be met by a measured concentration of SO2 gas and CO2 gas in exhaust gas.SO2 gas concentration (ppm)÷CO2 gas concentration (vol %)<4.3  [Math. 1]
A laser-type gas analyzer such as the above-described one can be used as a means for measuring the concentration of SO2 gas and CO2 gas. However, most conventional laser-type gas analyzers can measure one type of gas to be measured per device, while laser-type gas analyzers that can detect the concentration of two or more types of gas, for instance CO+CO2, NH3+H2O, HCl+H2O, or the like, are limited as regards the types of gas. Two laser-type gas analyzers have been conventionally necessary to measure the concentration of SO2 gas and CO2 gas, as in marine exhaust gas.
The reasons for this are explained next.
The optical absorption spectrum of the SO2 gas lies in the mid-infrared region. For instance, FIG. 24 is the optical absorption spectrum of SO2. A quantum cascade laser or the like that emits laser light of a wavelength of a mid-infrared region can be conceivably used, as the laser light source, in order to detect such an optical absorption spectrum.
The optical absorption spectrum of CO2 gas is in the near-infrared region. For instance, FIG. 25 is the optical absorption spectrum of CO2. A semiconductor laser or the like that emits laser light of a wavelength of a near-infrared region can be conceivably used, as the laser light source, in order to detect such an optical absorption spectrum.
Two laser-type gas analyzers having different laser light sources are thus required. The cost of the analyzers and construction costs increase as a result. This is compounded with the problem of the increased equipment size. Compact laser-type gas analyzers that measure the concentration of both SO2 gas and CO2 gas in a single device have become thus a necessity.
Marine exhaust gas comprises water and soot (dust). The influence of light amount attenuation by dust can be corrected by resorting to the conventional technology disclosed in Patent literature 2, such that the gas concentration can be accurately measured even if dust is hypothetically present in the flue.
For instance, a wavelength range scannable by the near-infrared laser element that is used encompasses light of a wavelength that is unaffected by absorption by CO2 gas having a spectrum such as the one illustrated in FIG. 25. By resorting to the conventional technique of Patent literature 2, therefore, the gas concentration can be measured accurately by correcting the received light amount using light of a wavelength that is unaffected by absorption by a gas component to be measured.
In SO2 gas having a spectrum such as the one illustrated in FIG. 26, however, the wavelength range that can be emitted by the mid-infrared laser element that is used does not include light of a wavelength unaffected by absorption by SO2 gas. Accordingly, DC-type absorption occurs due to the gas to be measured. This was problematic in that, with a light amount decrease due to dust being of DC type, it was difficult to discriminate between absorption by the gas to be measured and light amount attenuation by dust, and to measure accurately gas concentrations by correcting received light amounts, when measuring a gas such as SO2 gas.
Further problems arise when water is present in the exhaust gas in a substantial amount. The optical absorption spectrum of water appears at multiple sites outside the optical absorption spectrum of SO2 gas, as the gas to be measured, in the mid-infrared region (FIG. 24) for measuring SO2 gas. FIG. 27 illustrates the optical absorption spectrum of water. The optical absorption spectrum of water lies in the mid-infrared region, as does that of SO2 gas. It is therefore very difficult to measure SO2 gas concentration by excluding the optical absorption spectrum of water.
Specifically, when the concentration of water in the space to be measured is high, the laser light that is emitted by the quantum cascade laser, as a laser light source, is affected also by water, besides the gas to be measured.
This is problematic in that the received light amount is attenuated due to such an influence. This feature will be explained next. FIG. 28 illustrates the levels of a light reception signal (in other words, received light amount) of instances of experimental assessment of the influence of absorption by water upon detection of SO2 gas with the optical absorption spectrum wavelength of SO2 gas set to about 7.2 μm.
If attenuation of the received light amount arises only as a result of the influence of dust, such attenuation can be corrected by resorting to the method disclosed in Patent literature 2. However, FIG. 28 reveals that that the received light amount becomes increasingly attenuated as the water concentration (volume concentration) rises. As a result, a problem arises thus in a conventional laser-type gas analyzer in that when water is present in the space to be measured, the measured value of the gas to be measured becomes attenuated, and the gas concentration cannot be measured accurately.
It has become thus necessary to nullify both the influence of dust and the influence of water in order to analyze the SO2 gas concentration and the CO2 gas concentration in marine exhaust gas.
Similar problems arise when measuring the gas concentration of a first gas to be measured in a mid-infrared region, such as SO2 gas, and the gas concentration of a second gas to be measured in a near-infrared region, such as CO2 gas. As a result, it has been necessary to remove the influence of dust and/or the influence of water.